Modulators are devices that convey data by changing an aspect of a carrier signal in response to a baseband signal. Generally, the baseband signal is representative of the data to be conveyed. Once the carrier signal is modulated using the baseband signal and transmitted over a medium to a receiver, a demodulator demodulates the carrier signal and ultimately recovers the original data.
Phase modulation is a commonly used modulation technique. Phase modulation involves varying the phase of the baseband signal to represent the data to be conveyed. There are generally two types of phase modulation: analog and digital. Analog phase modulation involves the use of a continuously variable baseband signal that modulates the phase of a carrier signal. Digital phase modulation involves the use of a baseband signal containing distinct levels that modulates a carrier signal. One form of digital phase modulation is binary phase or “bi-phase” modulation, wherein the binary baseband signal comprises two phases, generally separated by 180 degrees. The phases represent the data to be transferred in the form of a sequence of 1 bits and 0 bits. For example, the phase shift could be 0 degrees for encoding a “0”, and 180 degrees for encoding a “1.” Similarly, the phase shift could be −90 degrees for encoding a “0”, and +90 degrees for encoding a “1.” The baseband signal may then be mixed with the carrier signal to generate a bi-phase modulated output, wherein the carrier signal “carries” the baseband signal to a demodulating device for demodulation.
The baseband signal used in a bi-phase modulators is typically generated by a driver. A driver may be capable of generating a baseband signal based on logic types such as low voltage differential signaling (LVDS), emitter coupled logic (ECL), low voltage positive emitter coupled logic (LVPECL), positive emitter coupled logic (PECL), etc., for example.
Various conventional ECL, PECL, LVPECL, and LVDS driver designs include the use of amplifiers, switches, and/or transistors. Typically, these conventional drivers are capable of providing a baseband signal having a frequency of 50 MHz to 100 MHz and a swing of 600 mV to 800 mV. Many conventional drivers are further characterized as producing a baseband signal that is return-to-zero. A return-to-zero signal is a signal that drops to zero between each bit. The zero between each bit is a neutral or rest condition, and is typically halfway between, for example, the phase shift for encoding a 1 bit and the phase shift for encoding a 0 bit. When described in the context of a phase modulation baseband signal, zero phase shift occurs during the rest condition.
Furthermore, many of these conventional bi-phase modulator driver designs are AC coupled. As a consequence, the output of the modulator driver will decay unless continually modulated at a sufficient modulation rate. The modulation rate in this context is dependent on the AC capacitors used in the modulator driver design.